1. Field of the Invention
The present invention relates to an electron emission device and, more particularly to a process for producing an electron emission device of the surface conduction type.
2. Related Background Art
Devices capable of emitting electrons with simple constructions have been known, such as a cold cathode device which has been proposed by M. I. Elinson et al. in Radio Eng. Electron Phys., vol. 10, pp 1290-1296, 1965.
This device makes use of a phenomenon in which electrons are emitted from thin film of a small area formed on a substrate, when electric current is made to flow through the film in parallel with the surface of the film. Electron emission devices relying upon this phenomenon are generally referred to as surface conduction type electron emission device.
Various types of surface conduction electron emission device have been proposed. For instance, the above-mentioned device developed by M. I. Elinson makes use of a thin film of the SnO.sub.2 (Sb). A device proposed by G. Dittmer (This Solid Film, Vol. 9, pp 317, 1972) uses an Au thin film, while a device proposed by M. Hartwell and C. G. Fonstad (IEEE Trans. ED Conf., pp 519, 1975) utilizes an ITO thin film. H. Araki et al (VACUUM. Vol. 26., No. 1. pp 22, 1983) proposes a device which incorporates a thin film of carbon.
FIG. 6 shows the construction of an example of such known electron emission devices of surface conduction type. The device has electrodes 1 and 2 for external electrical connection, a thin film 3 made of an electron emission material, and a substrate 5. An electron emitting region is denoted at 4.
Before put into use, a surface conduction type electron emission device is usually subjected to a heat treatment generally referred to as "electroforming" in which electric current is supplied to the device to form the electron emitting region. More specifically, a voltage is applied between the electrodes 1 and 2 so that electric current flow-through the thin film 3. As a result, the thin film 3 generates Joule heat which locally destructs, deforms or denaturates the thin film 3 so that a portion of the thin film 3 is changed to a state with a high electrical resistance and is formed to serve as the electron emitting portion 4, whereby an electron emitting function is obtained.
The state with high electrical resistance means a state in which minute cracks appears, generally ranging between 0.1 .mu.m and 5 .mu.m with structural discontinuity, i.e., so-called island structure, in these cracks. In such an island structure, fine particles of particle sizes ranging between several tens of Angstroms (.ANG.) and several micro meters (.mu.m) exist in a spatially discontinuous but electrically continuous state.
In operation, a voltage is applied between the electrodes 1 and 2 so that electrical current is supplied to the discontinuous film of high electrical resistance so as to flow in the surface region of the device, thereby causing the fine particles to emit electrons.
Thus, the known electron emission device has the electron emitting region 4 which is produced by the forming effected on the thin film 3 by heat generated as a result of a supply of electric current to the thin film 3. This known electron emission device suffers the following problems:
(1) Intentional design of the island structure is materially impossible, which makes it difficult to improve the device and causes a fluctuation in the quality of the device. PA0 (2) Island structures are unstable and cannot withstand a long use. In addition, the device tends to be destroyed by external electromagnetic noise. PA0 (3) The substrate tends to be damaged by large heat input incurred during execution of the forming process. This makes it difficult to produce a multi-staged device composed of a plurality of unit devices. PA0 (4) Only materials having comparatively small work function, e.g., gold, silver, SnO.sub.2 and ITO are usable as the island material, so that the device cannot produce a large output electrical current.
For these reasons, the surface conduction type electron emission devices, despite their simple construction, could not be satisfactorily put into industrial use.